Closed-Loop Polarization Mode Dispersion Mitigation for Fibre-Optic Time and Frequency Transfer

A polarization switching pulse interleaver is shown to be effective in reducing timing noise due to polarization mode dispersion in time and frequency transfer based on mode-locked lasers and standard single-mode fibres. In closed-loop time transfer over a 30-km dispersion-compensated fibre link with 300~fs of differential group delay, polarization interleaving reduced the delay variations to <20fs. The results indicate that the remaining drift is caused by polarization-dependent loss and by AM-to-PM noise conversion in the photodiodes, suggesting the need for a "double-balanced" phase detector in the receiver, that is, a phase detector balanced in power and polarization. By mitigating the polarization dependence, this work demonstrates a simple approach that can potentially yield sub-femtosecond-level, long-term time transfer in long-haul fibre links utilizing standard single-mode fibres.